A drill chuck normally has a chuck body rotatable about a chuck axis and provided at its front side with a plurality of axially forwardly projecting and radially displaceable jaws. A tightening ring rotatable on the body about the axis thereof is connected via appropriate screwthread formations to the jaws so that when this ring is rotated on the chuck body about the axis in a tightening direction the jaws are displaced radially toward each other and when rotated in an opposite loosening direction the jaws are displaced apart.
Normally the axially forward edge of the tightening sleeve or ring is formed with an array of teeth and the chuck body is formed with a radially outwardly open bore spaced slightly ahead of this array of teeth. The tip of a chuck key can be inserted in this bore so that teeth on the chuck head mesh with the teeth of the tightening ring. The chuck key can then be rotated to bring considerable torque to bear on the tightening ring and make the chuck very tight. Normally the tightening ring is a heavy cast element formed with the gear teeth for cooperation with the chuck key and with the screwthreads thereof for moving the jaws. The tightening sleeve that is fixed to this is a much lighter sheet-metal element and serves mainly to allow fast opening and closing of the chuck by hand.
When such a drill chuck is subjected to considerable vibration, as in a hammer drill, it can frequently loosen. Complex ratchet devices have been proposed to counter this tendency. The extra cost of these arrangements is normally excessive, and they also normally allow a small amount of loosening until the mechanism locks in a ratchet position.